Garpenbergite is a new mineral (IMA2020-099) from the Garpenberg Norra mine, Hedemora, Dalarna, Sweden. It occurs with carlfrancisite and minor stibarsen, paradocrasite and filipstadite in a fractured skarn matrix of granular jacobsite, alleghanyite, kutnohorite and dolomite. Crystals are short-prismatic, up to 1.5 mm in length. They have a blackish to greyish brown colour, and are lustrous semi-opaque, with brown streak. Garpenbergite is brittle, with an uneven to subconchoidal fracture. Cleavage is distinct on {010}. Hardness ≈ 5 (Mohs) and VHN100 = 650(40). Dcalc = 4.47(1) g⋅cm−3, overall ncalc = 1.85. Maximum specular reflectance values (%) obtained are 9.2 (470 nm), 9.1 (546 nm), 9.0 (589 nm) and 8.9 (650 nm). The empirical chemical formula of garpenbergite, based on electron microprobe data, is (Mn2+3.97Mg1.48Mn3+0.26Zn0.29)Σ6.00(As0.89Fe3+0.04Mn3+0.06Si0.01)Σ1.00(Sb0.98Fe0.02)Σ1O10[(OH)1.99Cl0.01]Σ2.00. The five strongest Bragg peaks in the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are 3.05 (30) (002), 2.665 (100) (161), 2.616 (40) (301), 2.586 (25) (251) and 1.545 (45) (462). The orthorhombic unit-cell dimensions (in Å) are a = 8.6790(9), b = 18.9057(19) and c = 6.1066(6), with V = 1001.99(18) Å3 for Z = 4. The crystal structure was refined from single-crystal X-ray diffraction data in the space-group Ibmm to R1 = 3.7% for 957 reflections. Garpenbergite, ideally Mn6As5+Sb5+O10(OH)2, is isostructural with manganostibite, Mn7AsSbO12, but possesses a cation vacancy (□) at an octahedrally coordinated structural site; the two minerals are thus related by the exchange Mn2+ + 2O2– → □ + 2(OH). The presence of hydroxyl groups is supported by vibration bands at 3647 and 3622 cm−1 in the Raman spectrum of garpenbergite and by bond-valence considerations.

Metal mining at Garpenberg, Hedemora, Dalarna (Dalecarlia), Sweden, has been described in annals since the fourteenth Century, however recent studies of Holocene lake-sediment records in the area indicate that small-scale extraction of copper occurred earlier, in the pre-Roman iron age, from ca. 375 B.C. (Bindler et al., 2017). Mining is still active; the current business has been operated since 1957 by Boliden AB, which presently produces 3 Mt of ore per year. The grades are 0.5 g/t Au, 70–160 g/t Ag, 0.1% Cu, 2% Pb and 3–4% Zn (Bolin et al., 2003; Tiu et al., 2021).

The sample containing the new mineral presented here was collected in situ at the 900 m level of the Garpenberg Norra mine (60°20′N, 16°13.5′E), where various exotic skarn mineral assemblages have been noted to exist (Eriksson and Kalinowski, 2001; Holtstam et al., 2001; Holtstam, 2002; Kolitsch and Holtstam, 2002; Nysten, 2003). Garpenbergite was first spotted by one of the authors (K.G.) in a polished section. Preliminary chemical data suggested a close relationship to manganostibite, ideally Mn7AsSbO12, a rare mineral first described by Igelström (1884) from the Nordmark ore field, Filipstad, Värmland, Sweden. A subsequent Raman study, however, indicated a distinct new species (cf. Holtstam, 2021).

The name of the mineral is for the place of discovery, Garpenberg. In the Middle Ages, ‘garp’ was the colloquial word in Swedish for a German person. The term here refers to the fact that the Swedish Crown recruited German miners at the time in order to increase know-how and develop regulations in the kingdom's growing mining industry (Carlberg, 1879).

The new mineral species and its name (symbol Grp) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2020-099, Holtstam et al., 2021). The holotype material is deposited in the type mineral collection of the Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden, under collection number GEO-NRM #20010351 (specimen and polished section) and #20200040 (polished section).

Garpenberg is situated in the northern part of the Palaeoproterozoic Bergslagen ore region in central Sweden, where numerous stratabound sulfide and (Fe±Mn) oxide deposits are hosted by 1.9–1.8 Ga felsic volcanic rocks and interlayered limestones, with enveloping siliciclastic sedimentary rocks and intruding Svecokarelian plutons (Stephens and Jansson, 2020). The Garpenberg polymetallic Zn–Pb–Ag–(Cu–Au) sulfide deposit stretches about seven km in a SW–NE direction, and comprises several ore bodies, mainly confined to a marble horizon. Significant mineralisation also occurs in adjacent metavolcanic rocks. At a stratigraphically lower level, several magnetite skarn ores are present. The rock units have experienced pervasive hydrothermal alteration proximal to the ore bodies (Vivallo 1985; Allen et al., 2003), metamorphism and polyphase deformation. Most or all of the Fe oxide and complex sulfide ores in the Garpenberg area formed within the time span 1895–1890 Ma during a syn-volcanic event (Jansson and Allen, 2011). Peak metamorphic conditions for the deposit are estimated at T ≥ 550°C and P < 0.35 GPa imposed around 1.85 Ga (Vivallo 1985).

The marbles at Garpenberg host enclaves of sporadic low-S, Mn–Zn–Pb–Ba–Sb–As mineralisation, reminiscent of both Långban (Holtstam and Langhof, 1999; Holtstam and Mansfeld, 2001) and Franklin–Sterling Hill (Dunn, 1995; Peck et al., 2009) types of deposits. The genetic relation to the main sulfide ores and skarns at Garpenberg remains unclear.

The size of the sample containing the new mineral is small, ca. 2 cm × 1.5 cm × 1 cm. Garpenbergite occurs in a filled fracture together with mainly carlfrancisite (Fig. 1). Minor stibarsen and paradocrasite appear in fractures of garpenbergite. Filipstadite has been observed in a few cases, as tiny overgrowths on garpenbergite. The rock matrix consists of granular jacobsite, alleghanyite, kutnohorite and dolomite. Following a brittle-deformation event, garpenbergite probably crystallised from a locally mobilised, As–Sb-rich fluid. Redox conditions were apparently varying, as evidenced by the presence of arsenate, arsenite and arsenide mineral species in the same fissure mineralisation. The mineral-forming episode is probably coeval with the transition from ductile to brittle deformation that occurred in the area at ~1.8–1.7 Ga (Stephens and Jansson, 2020).

The new mineral occurs as heavily fractured, short-prismatic (along [010]) and mostly subhedral crystals up to 1.5 mm in length. The macroscopic colour is blackish to greyish brown, the streak is light brown. Garpenbergite has a sub-adamantine to greasy lustre and is semi-opaque (translucent in thin splinters). The hardness is ~5 on the Mohs scale. The micro-indentation hardness (VHN100), obtained by means of a Shimadzu type-M tester, is 650 (6.37 GPa) based on ten measurements, giving a total range of 580–694. The shape of indentations are straight to slightly concave, with simple shell fractures (Jambor and Vaughan, 1990). Garpenbergite is brittle, with an uneven to subconchoidal fracture. Cleavage is distinct on {010}.

The density was not measured, because of current prohibition of heavy liquids; a calculated value of 4.47(1) g⋅cm−3 was obtained based on the empirical formula and unit-cell volume from single-crystal diffraction data (see below). Garpenbergite is non-magnetic with respect to a neodymium hand magnet, and soluble in 30% HCl (aq) at room temperature. No fluorescence was detected under ultra-violet light.

The optical character in transmitted light could not be determined owing to strong absorption and high refraction of the crystals. The overall refractive index (n) is calculated to 1.847, using Gladstone-Dale constants listed by Mandarino (1981). In reflected polarised light, garpenbergite is grey, with no perceptible bireflectance or pleochroism. It is weakly to moderately anisotropic under crossed polars. Reflectance spectra were measured in air with an AVASPEC-ULS2048×16 spectrometer connected to a Zeiss Axiotron UV-microscope (10×/0.20 Ultrafluar objective), using a tungsten filament lamp (100 W) and a dichroic HNPB polaroid filter. Silicon carbide (Zeiss no. 846) was used as standard, and the annular measurement area was ~0.1 mm in diameter on the sample surface. Results from the visible spectrum range (average of 100 scans, 100 ms integration time) are given in Table 1.

In comparison with the related manganostibite, garpenbergite is slightly more translucent, and has a lower overall reflectance.

A micro-Raman spectrum of garpenbergite (Fig. 2) was obtained from a crystal in the polished section on a LabRAM HR 800 spectrometer, using a 514 nm Ar-ion laser source at < 1 mW power, a Peltier-cooled (–70°C) 1024 × 256 pixel CCD detector, a Olympus M Plan N 100×/0.9 NA objective and laser spot of ca. 3 μm. A 600 grooves/cm grating was used, and the spectral resolution is ~1 cm–1. Peak positions were corrected against the Raman band at 789 cm–1 of a silicon carbide crystal. Instrument control and data acquisition (range 100–4000 cm–1, 10 s exposure time in 20 cycles) were made with the LabSpec 5 software. Laser-induced degradation of the sample was not observed. A single crystal of manganostibite (from Igelström's type specimen, GEO-NRM #18840177) was measured with the same settings for a comparison (Fig. 2).

Observable bands are at 3647, 3622, 1300, 1020, 818, 791 (shoulder), 695, 659, 620 (shoulder), 580, 541, 512, 482, 464, 339, 259 and 233 cm−1. The two distinct Raman bands above 3600 cm−1 are related to OH-stretching vibration modes. The strongest peak at 818 cm−1 (with shoulder at 791 cm−1) corresponds to stretching vibrations of the [AsO4]3− groups (Myneni et al., 1998). Bands in the region 695–620 cm−1 are attributable to the symmetric and antisymmetric stretching modes of SbO6 octahedra (cf. Guillén-Bonilla et al., 2014), whereas peaks at lower frequencies down to ~450 cm−1 probably arises from the Mn(Mg)–O stretching in tetrahedra and octahedra. Specifically, bands around 530 cm−1 could be attributed to octahedrally coordinated Mn2+ (Bernardini et al., 2021). In the spectrum of manganostibite, corresponding bands occur at similar positions (within 10–20 cm−1), with a clear exception for the OH vibration modes, which are absent.

Preliminary energy-dispersion (EDS) X-ray measurements on a polished sample section showed Mn, Mg, Sb, As, Zn and O to be present in garpenbergite. The chemical composition was ultimately determined using a JEOL JXA 8230 electron microprobe operating in wavelength dispersive spectroscopy mode at 15 kV and 20 nA, with a defocused beam size of 10 μm (to minimise diffusion of elements mobile under the electron beam). Measurement time on peaks and background was 20 and 10 s, respectively. Reference materials and measured lines were: rhodonite (SiKα), orthoclase (AlKα), Fe2O3 (FeKα), MnTiO3 (MnKα), diopside (MgKα and CaKα), adamite (AsLα and ZnKα), InSb (SbLα), fluorite (FKα) and tugtupite (ClKα). The number of spot analyses was 49. The H2O content was not determined directly, because of scarcity of material. No CO2 was indicated by crystal structure refinements or spectroscopic data. The following elements were sought but found to be below detection: Na, Ca, Al, P, V and F. The results of the analyses are given in Table 2. Associated carlfrancisite was also analysed (10 spots) at the same analytical conditions, and gave: SiO2 8.41, Al2O3 0.12, Fe2O3 0.77, MnO 51.84, MgO 10.87, CaO 0.03, As2O5 18.23, Sb2O5 0.06, ZnO 0.42, F 0.11, H2Ocalc 8.31, total 99.17 (all in wt.%).

The empirical formula of garpenbergite, calculated on the basis of eight cations and assuming – as inferred from crystal-structural features – As and Sb as pentavalent cations, and with the Mn2+/Mn3+ ratio adjusted to obtain overall electro-neutrality, is (Mn2+3.97Mg1.48Mn3+0.26Zn0.29)Σ6.00(As0.89Fe3+0.04Mn3+0.06Si0.01)Σ1.00(Sb0.98Fe0.02)Σ1.00O10[(OH)1.99Cl0.01]Σ2.00. The ideal formula becomes Mn2+6□As5+Sb5+O10(OH)2, which corresponds to (in wt.%) MnO 59.10, Sb2O5 22.44, As2O5 15.96, H2O 2.50, total 100.00.

The empirical formula for associated carlfrancisite, based on 96 anions with (OH, F) = 42 atoms per formula unit (apfu) and As3+ = 2 apfu, is Mn32+(Mn2+30.42Mg12.33Zn0.24Ca0.02)Σ43.01(As3+O3)2(As5+O4)4[(Si0.80As5+0.11Fe3+0.06Sb0.02Al0.02)Σ1.01O4]8(OH0.98F0.02)42.

Powder X-ray diffraction data (Table 3) were obtained with a Bruker D8 Venture diffractometer equipped with Photon III CCD detector and using copper radiation (CuKα, λ = 1.54138 Å), with 600 s of exposure; the detector-to-sample distance was 7 cm. The program Apex3 (Bruker AXS Inc., 2016) was used to convert the observed diffraction rings to a conventional powder diffraction pattern. A least-squares refinement of the orthorhombic unit cell gave the following values: a = 8.6919(10) Å, b = 18.927(3) Å, c = 6.1110(6) Å and V = 1005.3(1) Å3 for Z = 4.

A single-crystal X-ray diffraction study was performed on a 55 μm × 45 μm × 40 μm crystal fragment (counterpart of the piece used to make polished section for electron probe analysis) using a Bruker D8 Venture diffractometer equipped with Photon III CCD detector, with graphite-monochromatised MoKα radiation (λ = 0.71073 Å), with 20 s exposure time per frame; the detector-to-sample distance was 60 mm (summarised in Table 4). The small fragment was mounted on a 5 μm diameter carbon fibre, which was, in turn, attached to a glass rod. The diffraction intensity data were integrated and corrected for standard Lorentz polarisation factors with the software package APEX3 (Bruker AXS Inc., 2016). A total of 1317 unique reflections was collected up to 2θ = 73.40°. The program SHELXL (Sheldrick, 2015) was used for the refinement of the structure. The structure solution was initiated from manganostibite structure data (Moore, 1970), and the structure solution was undertaken in the space group Ibmm.

Though it has a nearly identical unit-cell geometry and structural topology with manganostibite (Moore, 1970), it was immediately realised that the octahedral position at the origin of the cell in manganostibite is empty in garpenbergite (see below), thus giving an overall content of M cations = 6, and not 7 as in manganostibite (cf. Table 5).

The site occupation factor (s.o.f.) at the cation sites was allowed to vary (Mn vs. Mg for the M sites and Sb vs. As for the other metal sites) using scattering curves for neutral atoms taken from the International Tables for Crystallography (Wilson, 1992). The refinement of the site-occupancies gave the scattering values and the possible site populations reported in Table 6 (unconstrained refinement). At this point, taking into account the observed bond distances (Table 7), the s.o.f. occurring at the M and Sb/As sites, and the electron microprobe analysis, the following site populations were proposed: Sb = Sb1.00; As = As0.89Fe0.06Mn0.04Si0.01; M1 = Mn0.71Mg0.22Zn0.07; M3 = Mn0.70Mg0.30; and M4 = Mn0.71Mg0.22Zn0.07 (Table 6). These proportions were then fixed in subsequent refinement cycles (see Table 6 – constrained refinement). The bond-valence sums (Table 5) indicate O1 as a likely hydroxyl ion in the structure. However, the individual H atoms could not be located in the difference-Fourier maps.

The crystal structure refinement finally converged to R1 = 3.7% for 957 reflections with F > 4σ(F) and 53 parameters. The experimental details and all R indices obtained are summarised in Table 4. Fractional atomic coordinates and isotropic displacement parameters are reported in Table 5. Bond distances are given in Table 7, and calculated bond-valence sums in Table 8. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Description of the crystal structure

The crystal structure of garpenbergite (Fig. 3a) consists of a cubic close-packed array of oxygen atoms with layers stacked along [130], with cations occupying tetrahedral and octahedral voids in a spinel-like arrangement. The heteropolyhedral framework consists of chains of edge-sharing M4 and Sb octahedra along the b axis, connected by strips of edge-sharing M3 octahedra stretching along c. M1 (×2) and As tetrahedra sharing corners with each other in triplets, and with adjacent octahedra. The calculated bond-valance sums (Table 8) for cation sites are in good agreement with the expected formal charges of the main ionic species, Mn2+, As5+ and Sb5+. The higher value obtained for the M3 polyhedron is probably related to accumulation of Mn3+ at that site, along with some enrichment of Mg. The octahedrally coordinated M2 site, filled with Mn2+ in manganostibite, is vacant in garpenbergite (Fig. 3b). Charge-compensation is achieved by incorporation of protons in the structure. This is most likely to occur close to the octahedral vacancy, in order to obtain a favourable electrostatic situation locally. As O1 and O4 are the most under-saturated anions in the structure (1.28 and 1.79 valence units, respectively), we conclude that protonation occurs along edges of the otherwise empty M2 octahedron. The hydroxyl-stretching bands around 3600 cm−1 suggest O⋅⋅⋅O contacts > 3.0 Å (Libowitzky, 1999), in agreement with O1–O4 distances of 3.2 and 3.3 Å obtained for the present crystal structure.

Relation with other mineral species and synthetic compounds

The structural topology of garpenbergite is nearly the same as for manganostibite (Moore, 1970). The new mineral is derived from manganostibite via the coupled ion exchange Mn2+ + 2O2− → □ + 2(OH), or in vector notation, Mn2+(□H+2)–1, which leaves the space-group symmetry invariant. Notably, Ibmm is one of the orthorhombic subgroups of the Fd$\bar{3}$forumlam spinel space group (Moore and Smith, 1969).

Garpenbergite fits in the Strunz group 4.CA rather than 4.BA (with manganostibite; Strunz and Nickel, 2001) due to a different metal:oxygen ratio (2:3 vs. 3:4). The synthetic orthorhombic phases denoted ‘spinelloid phase II’ (e.g. Ni4Al4SiO12, Ma, 1974; Fe7.71Si1.29O12, Angel and Woodland, 1998) and Co7As2O12 (Barbier, 1997) have a close structural kinship with both manganostibite and garpenbergite. Moreover, the nesosilicate gerstmannite (Moore and Araki, 1977a) has a metrically similar unit cell, but crystallises in the space-group Bbcm. Somewhat related are also the OH-bearing arsenosilicates holdenite (Moore and Araki, 1977b) and kolicite (Peacor, 1980). Note that the overgrowth of filipstadite on garpenbergite is probably epitaxial; both minerals exhibit the spinel-structure theme with Sb5+ ions ordered at octahedral sites, and the unit-cell parameter a of garpenbergite is ⅓ × a of filipstadite (~26 Å; Bonazzi et al., 2013).

The handbook formula for garpenbergite might be written in simplified form as Mn6AsSbO10(OH)2, but at this instance, we prefer Mn6□AsSbO10(OH)2 to emphasise the close relationship with manganostibite. If the essential constituents are grouped after coordination environment (M = octahedral and T = tetrahedral), the general formula is M6O2T3O10 for manganostibite and M5□(OH)2T3O10 for garpenbergite. These minerals are part of a homologous series formalised as M2nφn–1TnO3n+1, where n = 1 for spinel and n = 3 for manganostibite/garpenbergite, and with φ representing O2– or OH.

Crystal chemistry

There is partial replacement of Mn by Mg and Zn in the ideal end-member formula of garpenbergite, which is apparent in the chemical point analytical data for the slightly inhomogeneous crystal (Fig. 4). It has a significantly higher Mg concentration than that known for manganostibite (1.0–2.9 wt.% MgO; Holtstam et al., 1998), but Mg2+ is not strongly ordered at any particular site in the structure. Garpenbergite is also comparably enriched in Zn; this element ought to be concentrated at the tetrahedral (M1) site. The M3 octahedron is axially compressed, with two short and four long M–O distances. This coordination environment is thus favourable for Mn3+ due to the Jahn–Teller interaction of the d4 ion.

The Sb content in garpenbergite is close to the nominal value of 1 apfu; the situation is invariably the same for different samples of manganostibite (Holtstam et al., 1998). In contrast, the As site is hosting additional cations, like Si, Fe3+ and probably Mn3+. This is not unexpected, since manganostibite is shown to contain up to 0.48 Si apfu (Holtstam et al., 1998) and explained by considerations of electrostatic bond valances for the ideal (non-substituted) structure (Moore 1970). The AsO4 tetrahedron may seem too small to accommodate the trivalent cations; however, limited Fe3+-for-As5+ substitution is still indicated by chemical variations in the crystal (Fig. 5).

The mechanism introducing H+ in a spinel-like atomic arrangement invoked here is not unique; it is found for, e.g. high-pressure variants of MgAl2O4 spinel (Bromiley et al., 2010) and related compounds, like ringwoodite (n = 1) (Grüninger et al., 2020) and wadsleyite (n = 2) (Deon et al., 2010), and is coupled to cation vacancies at, most frequently, octahedral sites in those structures. For garpenbergite, high-pressure conditions are not inferred, but a high-${\it f}_{{\rm H}_ 2{\rm O}}$forumla environment of formation is indicated by the close association with the strongly hydrous mineral carlfrancisite. Notably, carlfrancisite from Garpenberg is very similar in composition to the holotype specimen from Kombat, Namibia (Hawthorne, 2018). We do not, however, expect to find the same paragenesis as described here at Kombat, because of insignificant Sb concentration in the ore lithologies there.

We thank Franz Vyskytensky, Kopparberg, for providing specimens from the Garpenberg deposit over the years. Oona Appelt (GFZ) is thanked for her support with the electron microprobe work. Reviews by two anonymous reviewers and Structural Editor Peter Leverett improved the paper.

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